For decreasing volume and weight, increasing power density, improving portability, and the like reasons, people in the industry have made every effort to increasing the density of semiconductor devices. One way to achieve this is using a 3D structure instead of a conventional 2D structure. A 3D semiconductor structure may comprise a plurality of stacks formed on the substrate. Theses stacks are separated from each other by high aspect ratio trenches or holes. Some structures may be formed in the trenches or holes along the sidewalls of the stacks and/or on the bottom of the trenches or holes. However, as the height of the stacks increases, some problems relating these structures may emerge. For example, such structures will be harder to form and keep their desired configurations.